1. Field of the Invention
The invention relates to synthetic polymeric resins and more particularly to a branching agent and its use to branch polycarbonate resins.
2. Brief Description of Related Art
Polycarbonates are well known polymers which have good property profiles, particularly with respect to impact resistance, electrical properties, dimensional rigidity and the like. These polymers are generally linear, but can be made with branched sites to enhance their properties in specific ways. Low levels of branching are generally incorporated into the resin by co-polymerizing into the polymer backbone a tri or higher functional reagent to yield a thermoplastic polycarbonate resin with enhanced rheological properties and melt strength which make it particularly suitable for such types of polymer processing procedures as the blow molding of large, hollow containers and the extrusion of complex profile forms.
Sufficiently higher levels of branching sites in the resin will cause resin chains actually to join to each other to form partially or fully crosslinked resin networks which will no longer be thermoplastic in nature and which are expected to exhibit enhancements, over corresponding linear resins, in certain physical properties and/or in their resistance to abusive conditions, such as exposure to organic solvents. A wide variety of means have been employed to produce crosslinking in polycarbonate resin. These generally involve the incorporation of a suitably reactive chemical group either into the resin chain at its time of manufacture or as an additive to the resin after manufacture, or both. These reactive groups and the reactions they undergo are generally dissimilar from those characteristic of polycarbonate resin itself and are therefore prone to have detrimental side effects on the physical and/or chemical properties of the polymer. The conventional test used to judge the success of these means for crosslinking is to observe the formation of gels due to the crosslinked material when a resin sample is mixed with a solvent, such as methylene chloride, in which normal linear polycarbonate resin is highly soluble.
The major advantage of branched polycarbonate is its relatively high melt strength, or, more particularly, its high ratio of low-shear melt viscosity to high-shear melt viscosity (R*). This high melt strength makes branched polycarbonate uniquely suitable for applications such as blow-molded waterbottles and automobile bumpers, where the structural integrity of the melted parison under the influence of gravity is critical.
At present, branched polycarbonate is produced commercially in an interfacial phosgenation process, using 1,1,1-tris(4-hydroxyphenyl-)ethane, or THPE, as the branching agent. Producing branched polycarbonate in this way has several disadvantages including the need for extensive cleanout of the reactor after a production run and the risk of "gel" contamination of higher melt flow homopolycarbonate products during handling of the polycarbonate powders. It would therefore be advantageous to develop a method of production for branched polycarbonate which would not require the use of the phosgenation reactor and associated powder handling systems.
Previously, attempts have been made to produce branched polycarbonate from a trifunctional phenolic branching agent, such as THPE, and homopolycarbonate using reactive extrusion mediated by a basic transesterification catalyst, such as tetraethylammonium acetate (TEAA) monohydrate, in which all the aforementioned ingredients were admixed prior to melt extrusion. These efforts were unsuccessful, producing material with low melt strength. Recently, branched polycarbonate has been successfully produced via the reactive extrusion method described above using the tetrafunctional phenolic branching agent, 2,2,5,5-tetra-(4-hydroxyphenyl-)hexane, in place of THPE. Unfortunately, this tetrafunctional branching agent is very costly to produce, thus limiting the economic viability of this approach.
The present invention includes a new method to prepare branched or crosslinked polycarbonate resin. This approach involves the use of a branching agent additive to the resin which has structure and reactivity very similar to that of the polycarbonate resin repeat unit itself. Thus, it offers the dual advantages of allowing the branch sites to be incorporated into standard linear polycarbonate resin subsequent to manufacture of the resin and of providing this branching or cross-linking by a method which produces residual structural groups in the final composition which are expected to be physically and chemically compatible with the polycarbonate resin.
The state of the art is represented by the disclosures found, for example, in U.S. Pat. Nos. 4,415,722; 4,474,999; 4,469,861; 4,550,155; 4,621,132; and 5,089,598, all of which are incorporated herein by reference thereto.